Advances in high speed data processing circuitry have brought about an ever increasing time sharing of circuits in data processing systems. Although fully time-shared electronic control of a telephone switching system brings about many system advantages such as increased flexibility, new and advantageous system services and uniformity of system design independent of system traffic handling capacity, such systems do have inherent problems which are not present in prior art telephone switching systems and which are not present to the same degree in other data processing systems.
For example, in prior art electromechanical telephone switching systems such as the well-known Bell System crossbar system, a plurality of control circuits, i.e., markers, are provided in sufficient numbers to serve the traffic requirements of the switching center. Each marker is time-shared by a large number of lines and trunks; however, in such arrangements the loss of a single marker or of a relatively few markers of the group results only in a reduction in system traffic handling capacity and such a loss is not fatal to system operation.
A telephone switching system is a "near real time" machine in that it must serve the lines and trunks terminating in the office without unreasonable delays. Furthermore, a telephone switching system must run continuously if customer satisfaction is to be achieved. That is, it is not possible to stop the service of a telephone office during the course of the day or night to effect changes in wiring or to effect repairs on the system. A telephone exchange must be prepared to handle calls continuously as serious personal emergencies which require the use of the telephone system cannot be scheduld to occur at times which are convenient to the telephone system.
The above limitations of machine operation with regard to near real time response of the system and the requirement that the system be continuously operable do not generally apply to other electronic data processing arrangements and, if they do apply, are not applicable to the same degree.
An example of an electronically controlled telephone switching system wherein a single central control is time-shared to control all of the lines and trunks of the office is shown in U.S. Pat. No. 2,955,165 which issued to W. A. Budlong, G. G. Drew, J. A. Harr on Oct. 4, 1960. In a system such as this a failure in the central control or any other of the time-shared units of the system will result not only in a reduction in traffic handling capacity but, rather, a complete failure of the system. Furthermore, mere mechanical switching of duplicate units in such a system is not sufficient as the problems of switching from regular to standby units are such that the traffic handling capacity of the system may be unreasonably interrupted during switching.
The usual methods of evaluating the performance of electromechanical telephone systems and data processing systems and the remedial actions taken in response thereto are also not adequate in a commercial electronic program controlled telephone switching system. Subscriber satisfaction is based upon consistent and uninterrupted service. Accordingly, every effort must be made to assure early detection of potential trouble, to avoid recurrence of troubles and, once a trouble has been detected, to rapidly diagnose and repair the defect in order to assure the availability of backup facilities in the case of further troubles.
It is an object of this invention to increase the dependability of electronic program controlled telephone switching systems.
It is another object of this invention to increase the maintainability of electronic program controlled telephone switching systems and to simplify the manual maintenance checks attendent such systems.
These and other objects of this invention are achieved in one specific illustrative embodiment wherein system control is generally by means of a "central processor" which comprises a central control and memory wherein there is stored data and a logical program for performing both the normal operating functions of a telephone switching system and the maintenance functions of such a system. The program is arranged to accomplish the normal functions of a telephone switching system on a near real time basis and to also perform the following maintenance functions:
A. A plurality of subsystem checks during the course of normal call processing;
B. Maintenance routines to detect and isolate trouble;
C. Switching of subsystems to remove faulty units from the active combination of equipments; and
D. A systematic diagnosis of the source of a detected system failure.
Normal call processing may be functionally divided into two general classes:
(1) The collection and transmission of data from and to lines, trunks, and service circuits; and
(2) The processing of such data.
The collection and transmission of data must be accomplished on a near real time basis; therefore, such work functions are accomplished repeatedly with a high degree of timing precision. The data processing, however, may be carried on with a lower degree of timing precision. Both the interleaving of the normal call processing and maintenance functions and the execution of the "near real time call processing tasks" are accomplished by means of a system interrupt hierarchy which includes a plurality of interrupt levels and a base level.
System dependability is increased by providing an "emergency action sequencer" within the central processor, and by providing emergency action program procedures. A program controlled telephone switching system is capable of correcting faults within the system by way of program sequences only if the central control is capable of properly processing program order words and of acquiring such words from the store. In the event of a detected trouble which involves a major element of the central processor and which trouble is not remedied by program actions, the "emergency action sequencer" is enabled. The emergency action sequencer is a wired configuration which is capable of completing its assigned "emergency actions" even though the central processor is unable to process and/or acquire program order words. The emergency actions rearrange system components to make the system operative by avoiding the system failure. In the event that the first emergency action rearrangement is not successful in restoring the system to normal operation, successive emergency rearrangement actions are undertaken.
Heretofore both telephone switching systems and data processing systems have employed certain duplicated units of equipment and have provided means for switching between regular and standby units in the event of failure of one or the other. Such arrangements, however, generally employ electromechanical or mechanical swiches for transferring operation from one unit to another. In the present system greater flexibility of operation is achieved by switching between duplicate units by electronic means which avoid the inherent introduction of noise and delay which are present in prior art systems.
Further, with the advent of high speed electronic switching of duplicate units there is a greater tendency upon the part of the system programmer to switch system operation between duplicated units and thereby exercise duplicated units in regular service. This mode of operation is advantageous as it is possible that a unit may exhibit a trouble condition in regular service but not in standby service.
System data processing capability is enhanced by a number of measures provided in our system. The central control is arranged to operate in a mode of operation which is termed "three-cycle overlap" herein. In this mode of operation the central control is concurrently performing work functions relating to three successive single cycle orders. That is, while it is completing the operational step of a first order it is receiving the next succeeding order and is generating and transmitting the code-address of the second succeeding order. The data processing capacity of our system is further enhanced by the provision of sequence circuits which extend the period for completing the operational step of an order beyond a single machine cycle. Certain sequence circuits momentarily halt the flow of program order words; however, others extend the degree of overlap between successive orders.
The program order structure adopted herein is designed to optimize the system data processing capacity and to efficiently use the information capacity of the program order words. Although most orders in the order structure are general purpose orders which find application in many facets of machine operation, there are, however, individual orders which are designed to be employed in conjunction with one or more other orders to efficiently perform routine tasks which the machine must repeatedly perform.
In accordance with one feature of our invention the major divisions of the system are duplicated and means are provided for rapidly switching duplicate elements into active and standby combinations of equipment.
In accordance with another feature of our invention access is provided to large numbers of test points within all of the major elements of our system.
In accordance with another feature of our invention an emergency action sequencer serves to effect rearrangements of active combinations of major elements of the central processor even though the currently active combination is unable to process and/or acquire program order words.